seeking for help about a room with negative pressure
 

hi guys,
i am trying a room with negative pressure. the details are: the pressure of this room is -620Pa, there exists a hole in some wall, and the pressure outside is 0, so the air flows into the room. to maintain the negative pressure, there exists an exhaust fan. so, how i simulate this problem to get the air field?
i set the hole as pressure inlet and the value is 0, right? the depth of the wall is 0.6m, and i also set an exhaust fan and the gauge value is -620Pa and this fan can increase the pressure of the exhuasted air by 620Pa, right?
but i am depressed by this simulation. the massflow rate is too big for my question and the pressure of the room is not always -620Pa. i am puzzled now.
so, have you some good suggestions for my question?
Ps: the hole in the wall is very very small. in fact, the question i am stuyding is the leakage of the concrete wall, so i set a hole in the wall.
thanks for your help!